1. Field of the Invention
This invention relates to a flow battery.
2. Description of the Prior Art
Flow batteries include such types as the fuel cell and the zinc halogen cell. There is also a flow-type storage battery, the redox flow battery, that uses fluid battery active material for both the positive and negative electrodes (positive electrode fluid and negative electrode fluid; collectively referred to as "electrode fluid").
Attempts have been made to improve the charge/discharge efficiency of flow batteries by reducing leakage current. Since leakage current consists mainly of current escaping via the electrolyte path, it can be suppressed by increasing the resistance of the leakage current path. One method that has been used for achieving this is to provide a plurality of lines (a manifold) for the flow of electrolyte in and out of the cells of the cell stack so as, in effect, to lengthen (and increase the resistance of) the leakage current path among the cells. Leakage current has also been reduced by completely subdividing all of the electrode fluid lines and providing each subdivision with a tank and a pump, thereby ensuring zero leakage current between subdivisions. From the viewpoint of structural simplicity and the increased safety from leakage current accidents that this structural simplicity provides, the latter method of completely subdividing the electrode fluid lines is preferable.
However, since the complete subdivision of the electrode fluid lines necessitates the provision of as many small pumps as there are subdivisions, this method is disadvantageous in that it greatly increases the amount of labor required for inspection, maintenance and repair, degrades reliability, and reduces overall pump efficiency by 10% or more. The lowering of the pump efficiency is a particular problem in the case of the flow storage battery, whose charge/discharge energy efficiency is affected on the order of from several percent to several tens of percent by the pump efficiency. A decrease in pump efficiency of 10% or more reduces the overall energy efficiency of the battery by several percent. A drop in energy efficiency of several percent is a major problem in secondary batteries, which are intended for the storage of large amounts electric energy.
The object of this invention is to provide a flow battery that overcomes the aforesaid problems of the prior art, more particularly to a flow battery that achieves improved charge/discharge coulomb efficiency and voltage efficiency, without reducing pump efficiency.